Angiotensins, bradykinin, and prostaglandins modulate renovascular resistance, glomerular filtration, and tubular sodium transport. Current theory suggests that these physiological actions are initiated by the binding of these hormones to specific receptors. In this protocol, we propose to identify and quantitate the receptor binding sites for these hormones in rat kidneys using conventional light microscope and electron autoradiography. The specificity of binding will be determined by using analogs of different biological potency; saturability of binding will be assessed by determing competition for the available binding sites between labelled and unlabelled hormone. In separate experiments, radioreceptor assays will be employed to study hormone binding to isolated glomeruli and tubules in vitro. The kinetics of hormone binding and degradation of these renal structures will be assessed, and receptor binding site affinities and concentration will be determined. The specificity, saturability and reversibility of binding will be characterized.